UFC 91 takes place Nov. 15 at the MGM Grand Garden Arena in Las Vegas and airs live on pay per view.

The Wrestling Observer first reported the replacement on Saturday.

As MMAjunkie.com (www.mmajunkie.com) reported on Friday, Riddle suffered a knee injury during a training session and will soon undergo a second MRI. It’s believed he may suffered an ACL injury or torn meniscus.

Brown, an Ohio-based fighter who advanced to the “TUF” quarterfinals after defeating Josh Hall and Jeremy May, eventually lost to show champion Amir Sadollah.

However, Brown rebounded at the show’s live finale and defeated “TUF 6″ veteran Matt Arroyo with a second-round TKO. He most recently fought in Atlanta at UFC 88, where he dropped a close split decision to a tough Dong Hyun Kim at the Sept. 6 event.

Thomas made his professional debut in April 2007. The 23-year-old worked his way through smaller organizations in the Midwest, including Courage Fighting Championships and Extreme Challenge, and he won nine of his first 10 bouts to earn a ticket into the UFC. He made his octagon debut in August at UFC 87 and suffered a second-round submission loss to Ben Saunders.

The Thomas-Brown fight will appear on the night’s un-televised preliminary card.

For the latest on UFC 91, which features a main event between UFC heavyweight champion Randy Couture and Brock Lesnar, check out the UFC Rumors section of MMAjunkie.com.